The pull of electrons is the main reason. The pull of the lone electron of hydrogen by oxygen shortens bond length; this is easily seen by looking at the electronegativity difference between oxygen and hydrogen. Although alcohol also contains a OH bond, the OH is bonded to a carbon making a longer bond. And the sharing of electrons between carbon and hydrogen causes the bond length to be longer than you would observe between oxygen and hydrogen.
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Water molecules are smaller and have less mass than alcohol and oil molecules. Explain why water is more dense than alcohol and oil.
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